What is the use of gold in Civilization 4?

I'm still new in Civilization 4 and had a lot of questions about playing the game. I'm finding the game to be insanely complicated so please keep that in mind while giving your answer. If you use too much game jargon I won't understand. Thanks for your help in advance.

Question: Coins. I have seen that if I decrease the research I can start saving coins in my treasury. But till now I haven't used them. What is the use of these coins? When do we make use of them really? As beginner should I always keep my researched maxed out (as long as coin consumption is not negative)?

Solution 1:

Coins are your money:

• You can trade them to other civilizations in exchange for anything: technologies, maps, resources, the other civilization going to war for you. (After researching the technology Currency)
• You can spend them to quickly finish new units and buildings. (depending on your civilization's current government)
• You can spend them to upgrade your units which have become obsolete into the newer better units. (But only inside your territory, or maybe even in cities, or in cities you have Barracks in. I can't recall which one sorry!)
• You spend them each turn to pay your units. If you're not earning enough coins each turn to pay your units, and you don't have enough in your treasury to cover the loss, your units will start disbanding. (Deleting themselves)
• You spend them each turn to pay for your cities to run. The more cities you have, and the further they are from your capital, the more they cost.
• You can effectively spend coins you've saved in your treasury on research, by setting your research high enough that you are losing coins. Just watch for your treasury running out!

I'm probably even forgetting a few. Just think of coin as money. What can you do with money? Spend it on things you have to, then spend it on anything you like if you have any left over!

If I've used any terms you're not familiar with, just ask for clarification in a comment.

Solution 2:

Coins, or gold pieces, are the only way to upgrade your combat units to more modern versions (i.e. Archer -> Longbowman). You can also use coins for trading with other civilizations, e.g. in exchange for technologies, resources, maps, pacts etc.

Furthermore, each new city founded by your Civ will cost you gold (in form of a maintenance), so if one day you got yourself stuck at around 0 gold, you may find you are not able to create new settlements without significant reduction of your technology/culture rates.

Coins are also required for combat units upkeep, so if your coins will get reduced to 0 with a negative income, then some of your valuable troops might simply desert.